Heat assisted magnetic recording with exchange coupling control layer

ABSTRACT

An apparatus includes a first magnetic layer. A second magnetic layer overlies the first magnetic layer and is magnetically softer than the first magnetic layer. An exchange control layer is between the first magnetic layer and the second magnetic layer. The exchange control layer is magnetic and increases vertical coupling between the first magnetic layer and the second magnetic layer.

SUMMARY

Provided herein is an apparatus including a first magnetic layer. Asecond magnetic layer overlies the first magnetic layer and ismagnetically softer than the first magnetic layer. An exchange couplingcontrol layer is between the first magnetic layer and the secondmagnetic layer. The exchange coupling control layer is magnetic andincreases vertical exchange coupling between the first magnetic layerand the second magnetic layer.

Also provided herein is an apparatus including a first magnetic layer. Asecond magnetic layer overlies the first magnetic layer and ismagnetically softer than the first magnetic layer. An exchange couplingcontrol layer is between the first magnetic layer and the secondmagnetic layer. The exchange coupling control layer includes a higher Ms(saturation magnetization) than the first magnetic layer and the secondmagnetic layer at the writing temperature.

Also provided herein is an apparatus including a write head operable toproduce a magnetic field. A first magnetic layer is also providedwherein the magnetic field is insufficient to change a magnetization ofthe first magnetic layer at room temperature. A second magnetic layeroverlies the first magnetic layer. The second magnetic layer ismagnetically softer than the first magnetic layer. An exchange controllayer is between the first magnetic layer and the second magnetic layer.The magnetic field is sufficient to change the magnetization of thesecond magnetic layer at a Tc (Curie temperature) of the exchangecontrol layer.

These and other features and advantages will be apparent from a readingof the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a heat assisted magnetic recording media including a hardmagnetic layer, a soft magnetic layer, and an exchange coupling controllayer according to one aspect of the present embodiments.

FIG. 2 shows the heat assisted magnetic recording media undergoing awriting process according to one aspect of the present embodiments.

FIG. 3 shows the heat assisted magnetic recording media after removal ofthe heat according to one aspect of the present embodiments.

FIG. 4 shows a heat assisted magnetic recording media including a hardmagnetic layer, a soft magnetic layer, an amorphous exchange couplingcontrol layer, and a granular exchange coupling control layer accordingto one aspect of the present embodiments.

FIG. 5 shows a graph illustrating the effect of temperature on Hk invarious layers of a heat assisted magnetic recording media according toone aspect of the present embodiments.

FIG. 6 shows a graph illustrating the effect of temperature on Ms invarious layers of a heat assisted magnetic recording media according toone aspect of the present embodiments.

FIG. 7 shows a graph illustrating delayed switching of the highest Tcgrains of the hard magnetic layer according to one aspect of the presentembodiments.

DESCRIPTION

Before various embodiments are described in greater detail, it should beunderstood that the embodiments are not limiting, as elements in suchembodiments may vary. It should likewise be understood that a particularembodiment described and/or illustrated herein has elements which may bereadily separated from the particular embodiment and optionally combinedwith any of several other embodiments or substituted for elements in anyof several other embodiments described herein.

It should also be understood that the terminology used herein is for thepurpose of describing the certain concepts, and the terminology is notintended to be limiting. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood in the art to which the embodiments pertain.

Unless indicated otherwise, ordinal numbers (e.g., first, second, third,etc.) are used to distinguish or identify different elements or steps ina group of elements or steps, and do not supply a serial or numericallimitation on the elements or steps of the embodiments thereof. Forexample, “first,” “second,” and “third” elements or steps need notnecessarily appear in that order, and the embodiments thereof need notnecessarily be limited to three elements or steps. It should also beunderstood that, unless indicated otherwise, any labels such as “left,”“right,” “front,” “back,” “top,” “middle,” “bottom,” “beside,”“forward,” “reverse,” “overlying,” “underlying,” “up,” “down,” or othersimilar terms such as “upper,” “lower,” “above,” “below,” “under,”“between,” “over,” “vertical,” “horizontal,” “proximal,” “distal,” andthe like are used for convenience and are not intended to imply, forexample, any particular fixed location, orientation, or direction.Instead, such labels are used to reflect, for example, relativelocation, orientation, or directions. It should also be understood thatthe singular forms of “a,” “an,” and “the” include plural referencesunless the context clearly dictates otherwise.

Terms such as “over,” “overlying,” “above,” “under,” etc. are understoodto refer to elements that may be in direct contact or may have otherelements in-between. For example, two layers may be in overlyingcontact, wherein one layer is over another layer and the two layersphysically contact. In another example, two layers may be separated byone or more layers, wherein a first layer is over a second layer and oneor more intermediate layers are between the first and second layers,such that the first and second layers do not physically contact.

Perpendicular magnetic recording (“PMR”) media may include a PMRexchange coupling control layer that separates softer and hardermagnetic layers. The PMR exchange coupling control layer is a weaklymagnetic or non-magnetic breaklayer that reduces vertical exchangecoupling between the soft and hard magnetic layers of the PMR structure.However, such a PMR exchange coupling control layer may be detrimentalto heat assisted magnetic recording (“HAMR”) media, wherein theswitching temperature during the writing process is far higher than theroom temperature writing process of PMR. Although the room temperatureanisotropy and thermal stability of HAMR recording layers is generallymuch higher than that of conventional PMR recording layers, at the onsetof switching in HAMR, magnetic moment (Ms), anisotropy energy (KuV), andvertical exchange coupling (J) are all lower than the comparable valuesduring switching in a conventional PMR recording process.

It has been unexpectedly discovered that, instead of decreasing verticalcoupling (as is generally desired for PMR), HAMR media instead benefitsfrom increasing vertical exchange coupling between softer and hardermagnetic layers. As such, a HAMR exchange coupling control layer mayinclude a higher saturation magnetization (“Ms”) and/or higher Curietemperature (“Tc”) than the hard recording layer of the HAMR media.

Referring now to FIG. 1, a HAMR media 100 including a hard magneticlayer 102, a soft magnetic layer 104, and an exchange coupling controllayer (“ECL”) 106 is shown according to one aspect of the presentembodiments. For clarity of illustration, other layers of the HAMR media100 are not illustrated, but are understood to be present (e.g.substrate, SUL, seed layer, heatsink, overcoat, etc.). The hard magneticlayer 102 is granular and includes hard layer magnetic grains 108 thatare separated by segregant (not shown). For example, in variousembodiments the hard layer magnetic grains 108 may include one or moreL10 FePt containing alloys.

The ECL 106 overlies the hard magnetic layer 102. The ECL 106 isgranular and includes ECL layer magnetic grains 110 that are separatedby segregant (not shown). For example, in various embodiments the ECLlayer magnetic grains 110 may include an L10 Fe—Pt based alloy, an Fe—Pdbased alloy, Co—Pt based alloy, Co—Pd based alloy, or FePtCo. Inaddition, the ECL 106 may be relatively thin with respect to the hardmagnetic layer 102 and the soft magnetic layer 104. For example, the ECL106 may be less than 2 nm thick in the vertical direction. In someembodiments the ECL 106 may be strongly magnetic. The ECL 106 caninclude segregant or not include segregant. The ECL 106 can be granularor amorphous. The ECL 106 can be a single layer or multiple layers. Eachlayer can comprise any of the four combinations of crystal or amorphous,segregant or no segregant. Each layer of the ECL 106 may include aCo-containing alloy, an Fe-containing alloy, an ordered alloy, apartially ordered alloy, a disordered alloy, etc. The segregant may be Cor a carbide, BN, boride, nitride, oxide, etc. or a combination thereof.

In some embodiments, the ECL 106 and the hard magnetic layer 102 mayinclude ordered L10 alloys. The ECL 106 may include a higherconcentration of Fe+Co than the hard magnetic layer 102, and the ECL 106may comprise Cu. The ECL 106 may also include an L10 alloy with a higherratio of Fe+Co:Pt than the bottom sublayer of the hard magnetic layer102 and the top sublayer of the soft magnetic layer 104 in directcontact to the ECL 106. In various embodiments, the hard magnetic layer102, the soft magnetic layer 104, and the ECL 106 all include orderedL10 alloys. In some embodiments of ordered L10 ECL 106, grain-to-grainvertical coupling uniformity and switching temperature uniformity areimproved by reducing and diluting segregant concentrations to increasegrain core moment and volume. In some embodiments of ordered L10 ECL106, grain-to-grain vertical coupling uniformity and switchingtemperature uniformity are improved by lowering ordering temperaturewith additions such as Ag and Cu so as to improve ordering and therebyuniformity of properties. In other embodiments, the soft magnetic layer104 and the ECL 106 include disordered Co-containing hcp or fcc alloys.ECL 106 has a higher Co % concentration, resulting in higher moment Msthan soft layer 104. Higher moment in ECL 106 may be achieved by lowerconcentration of segregant as well as lower concentration of common hcpCo-alloy grain moment diluting elements including Cr, Pt, Ru, Cu. Infurther embodiments, ECL 106 comprises an Fe—Co alloy includingamorphizing agents such as Ta, B, W, Cr, and Nb forming a thin, highmoment, low thermal conductivity layer that further provides a templatefor continued growth of an hcp <0001> oriented Co-alloy soft layer 104.

As discussed above, the ECL 106 increases vertical exchange couplingbetween the hard magnetic layer 102 and the soft magnetic layer 104. Invarious embodiments, the ECL 106 includes a higher Ms and/or higher Tcthan the hard magnetic layer 102 and the soft magnetic layer 104 at theonset temperature of the HAMR recording process. As a result, exchangespring strength is increased between the hard magnetic layer 102 and thesoft magnetic layer 104.

The soft magnetic layer 104 overlies the ECL 106. The soft magneticlayer 104 is granular and includes soft layer magnetic grains 112 thatare separated by segregant (not shown). For example, in variousembodiments the soft layer magnetic grains 112 may include CoCrPt. Suchhcp or fcc Co-containing alloy soft layer 104 will generally besignificantly magnetically softer than an L10 ordered Fe—Pt based hardmagnetic layer 102. In other various embodiments, the soft magneticlayer 104 may also comprise an ordered L10 Fe—Pt containing alloy thatis softened by reduced ordering, for example by replacing Fe and/or Ptwith an element like Co, Cu, Ru, Ag, or even Fe or Pt to move off ofstoichiometry.

For clarity of illustration, the magnetizations of the hard layermagnetic grains 108, the ECL layer magnetic grains 110, and the softlayer magnetic grains 112 are depicted by up and down arrows. Therefore,the hard layer magnetic grains 108, the ECL layer magnetic grains 110,and the soft layer magnetic grains 112 are perpendicularly magneticallyoriented, as depicted by the up and down arrows. The magnetic easy axisof the hard layer is vertical as well, and that it may be preferable butis not necessary that the magnetic easy axis of the soft layer also bevertical, as long as the applied field and the room temperature exchangecoupling are both sufficient to maintain the soft layer magnetization ina vertical orientation. It will be further described that the ECL 106will in some embodiments control the soft layer easy axis orientation aswell as the vertical coupling strength between hard layer 108 and softlayer 104. It is understood that magnetization orientations may also bereferred to as positive (+), negative (−), north pole, south pole, etc.However, it is understood that such magnetic representations aresimplifications indicating, for example, the general location from whichmagnetic field lines emerge and reenter.

As illustrated, the hard layer magnetic grains 108, the ECL layermagnetic grains 110, and the soft layer magnetic grains 112 formgranular columns. The granular columns are separated by boundaries ofsegregant (not shown) that are non-magnetic spacers. In variousembodiments, the boundaries may be, for example, one of a combination ofoxides, carbides, borides and nitrides (e.g. SiO₂, TiO₂, B₂O₃, BN, C,etc.). The boundaries segregate the granular columns by physicallyseparating and therefore magnetically decoupling the granular columnsfrom each other. As such, the granular columns are parallel andhorizontal to each other with respect to overlying and underlyinglayers, and the magnetization of the granular columns is perpendicularto the overlying and underlying layers.

Referring now to FIG. 2, the HAMR media 100 undergoing a writing processis shown according to one aspect of the present embodiments. It isunderstood that for clarity of illustration, only a portion of the HAMRmedia 100 is depicted. A write head (not shown) produces and applies amagnetic field 214 and heat 216 to portions of the hard magnetic layer102, the soft magnetic layer 104, and the ECL 106. As illustrated, invarious embodiments the magnetic field 214 affects a larger area of theHAMR media 100 than the heat 216.

In response to the magnetic field 214, the soft magnetic layer 104 andthe ECL 106 change magnetization from up to down at a range oftemperatures below their Curie temperatures that includes the range ofswitching temperatures between onset of switching and below freezing ofthe hard layer 108, often including temperatures such as roomtemperature. Onset of switching refers to the highest temperature atwhich the first grains of the recording hard layer can be irreversiblyswitched by the combination of Zeeman energy from an applied field andany torque applied to the hard layer through exchange coupling ormagnetostatic forces. The onset temperature is generally a few degreesbelow Tc of the highest Tc grains of the hard layer, so that thosegrains have some moment for Zeeman energy to act upon and some Hk toprovide stability (KuV) to avoid rapid thermal reversal (erasure). Thefreezing temperature is the lowest temperature where the lowest Tcgrains of the hard layer are far enough below their Tc that their Hk hasincreased significantly above the applied field so that the combinationof switching forces is no longer able to rotate the magnetization of anyhard layer magnetic grains.

However, the magnetic field 214 is insufficient to change themagnetization of the hard magnetic layer 102 at room temperature or anytemperature below the freezing temperature of the hard layer. On theother hand, the heat 216 affects a smaller portion of the HAMR mediathan the magnetic field 214. In the area affected by the heat 216, themagnetic anisotropy field (“Hk”) and the Ms of the magnetic grains arereduced as the magnetic grains approach their Tc. In some embodiments,the soft layer magnetic grains 112 and the ECL layer magnetic grains 110have a higher Tc than the hard layer magnetic grains 108. As such, atthe Tc of the hard layer magnetic grains 108 the Hk and Ms of the hardlayer magnetic grains 108 will be reduced to zero while the ECL layermagnetic grains 110 and the soft layer magnetic grains 112 still exhibitmagnetic properties. As the hard layer magnetic grains 108 continue tocool below Tc, the Hk and Ms of the hard layer magnetic grains 108 beginto increase. While the Hk and Ms of the hard layer magnetic grains 108are still low, the thin ECL layer magnetic grains 110 increase theexchange coupling torque between the switched soft layer grains 112 andthe unswitched hard layer grains 108; and thereby increase the drivingforce to switch the magnetization direction of the hard layer magneticgrains 108 and increase the thermal stability of the hard layer magneticgrains 108. As such, the ECL layer magnetic grains 110 help to ensurethat the hard layer switching occurs and helps to avoid thermalun-switching after the switch happens. For example, in FIG. 2 switchingis depicted by the hard layer magnetic grains 108 switching from an uparrow to a down arrow, and un-switching is understood to refer to aninstance of the down arrow flipping back to an up arrow. Therefore, invarious embodiments the magnetic field 214 along with the increaseddriving force of the exchange coupling through the ECL 106 aresufficient to change the magnetization of the hard magnetic layer 102just below the Tc of the hard layer 102. As the hard layer magneticgrains 108 continue to cool, the Hk and Ms of the hard layer magneticgrains 108 continue to increase until the hard layer magnetic grains 108reach a freezing temperature, at which point Hk is so high that themagnetization direction of the hard layer magnetic grains 108 is frozenand will not change in response to the magnetic field 214. The freezingtemperatures of the soft layer 104 and the ECL 106 are generally muchlower than that of the hard layer and do not play a major role in thefreezing process.

Referring now to FIG. 3, the HAMR media 100 after removal of the heat isshown according to one aspect of the present embodiments. After removalof the heat 216 (FIG. 2), the previously heated magnetic grains 318 cooland are locked into the direction of the magnetic field 214 (FIG. 2). Asthe heat 216 and the magnetic field 214 are removed, the unheated softlayer magnetic grains 312 realign to the unheated hard layer magneticgrains 308.

Referring now to FIG. 4, a HAMR media 400 including a hard magneticlayer 402, a soft magnetic layer 404, an amorphous ECL 405, and agranular ECL 406 is shown according to one aspect of the presentembodiments. For clarity of illustration, other layers of the HAMR media400 are not illustrated, but are understood to be present (e.g.substrate, SUL, seed layer, heatsink, overcoat, etc.). The HAMR media400 is similar to the HAMR media 100, with the addition of the amorphousECL 405.

The hard magnetic layer 402 is granular and includes hard layer magneticgrains 408 that are separated by segregant (not shown). For example, invarious embodiments the hard layer magnetic grains 408 may include FePtor an alloy thereof. In various embodiments, the hard layer magneticgrains 408 may include an ordered L10 Fe—Pt containing alloy that may besoftened by reduced ordering, for example by replacing Fe and/or Pt withan element like Co, Cu, Ru, Ag, or even Fe or Pt to move off ofstoichiometry.

The amorphous ECL 405 is a continuous magnetic layer that overlies thehard magnetic layer 402. In various embodiments the amorphous ECL 405may be a Co-containing alloy that may further include Fe, Pt, andamorphizing agents (e.g. Ta, W, B, Nb, Zr, etc.). The amorphous ECL 405may have a saturation magnetic moment up to about 1.5T and have athickness less than about 3 nm. The amorphous ECL 405 forms an amorphousgrowth template on the hard magnetic layer 402.

The optional granular ECL 406 overlies the amorphous ECL 405. Thegranular ECL 406 is granular and includes ECL layer magnetic grains 410that are separated by segregant (not shown). For example, in variousembodiments the ECL layer magnetic grains 410 may include FePtCo. Invarious embodiments, the ECL layer magnetic grains 410 may include anordered L10 Fe—Pt containing alloy that may be softened by reducedordering, for example by replacing Fe and/or Pt with an element like Co,Cu, Ru, Ag, or even Fe or Pt to move off of stoichiometry. In addition,the granular ECL 406 may be relatively thin with respect to the hardmagnetic layer 402 and the soft magnetic layer 404. For example, thegranular ECL 406 may be less than 2 nm thick in the vertical direction.The granular ECL 406 has moment Ms higher than that of soft layer 404 orhard layer 402.

The soft magnetic layer 404 overlies the granular ECL 406. The softmagnetic layer 404 is granular and includes soft layer magnetic grains412 that are separated by segregant (not shown). For example, in variousembodiments the soft layer magnetic grains 412 may include CoCrPt. Suchhcp or fcc Co-containing alloy soft layer 404 will generally besignificantly magnetically softer than an L10 ordered Fe—Pt based hardmagnetic layer 402. In various embodiments, the soft magnetic layer 404may also comprise an ordered L10 Fe—Pt containing alloy that is softenedby reduced ordering, for example by replacing Fe and/or Pt with anelement like Co, Cu, Ru, Ag, or even Fe or Pt to move off ofstoichiometry.

Referring now to FIG. 5, a graph 500 illustrating the effect oftemperature on Hk in various layers of a HAMR media is shown accordingto one aspect of the present embodiments. The graph 500 illustrates someembodiments wherein the hard magnetic layer 102 (FIG. 1) includes a highHk at room temperature, as indicated by the line Hk_1 Hard 502, that ishigher than both the soft magnetic layer 104 (FIG. 1) and the ECL 106(FIG. 1). Both the soft magnetic layer 104, as indicated by line Hk_3Soft 504, and the ECL 106, as indicated by line Hk ECL 506 include alower Hk than the hard magnetic layer 102. FIG. 5 shows a case where ECL106 has lower Hk at room temperature than the soft magnetic layer 104,but these relative values can be reversed in various embodiments. Thewidth of the line Hk_1 Hard 506 schematically indicates that the hardlayer grains 108 have a range of values of Tc that result in a range ofswitching onset and freezing temperatures. Such variation reduces theability to get all grains to initiate and complete the switching processin a selected, desired narrow temperature window, thereby reducingtransition sharpness and limiting recording density. The variousembodiments of the ECL enable adjustment of this switching temperaturewindow to improve recording performance. Some embodiments provide higheronset temperature, enabling a longer time for more complete switchingbased on the same fixed temperature cooling profile defined by the head.Some embodiments further delay the onset of the highest Tc hard layergrains relative to the lower Tc hard layer grains, thereby reducing theeffect of grain to grain variation of Tc (sigma Tc), thereby narrowingthe switching temperature window and enabling increased transitionsharpness.

The graph 500 also illustrates that in some embodiments the hardmagnetic layer 102 includes a Tc, as indicated by Tc1, that is lowerthan both Tc3 of the soft magnetic layer 104 and Tc(ecl) of the ECL 106.FIG. 5 shows a case where Tc_2 ECL is higher than Tc3 soft, but theserelative values can be reversed in various embodiments.

It is understood that in various embodiments the Hk of the ECL 106 maybe higher or lower than the Hk of the soft magnetic layer 104. Someembodiments employ a higher Hk of the ECL 106, as that can correspond tohigher Ku, exchange stiffness, and vertical coupling strength.

In further embodiments, hard magnetic layer 102 and/or the soft magneticlayer 104 may include at least two sublayers. The sublayers may includedifferent magnetic values (e.g. Tc, sigma Tc, Ms, etc.). In someembodiments, the ECL may include a less than 3 nm amorphous layer havingMs between 700-1400 emu/cc and a less than 3 nm crystalline hcp layerhaving Ms between 700-1400. In some preferred embodiments amorphous orcrystalline layers or sublayers of ECL 106 are less than 2 nm thick.

In the presence of an applied magnetic field (not shown), switching ofthe magnetization in the hard magnetic layer 102 begins at a temperaturebelow Tc1, as indicated by vertical line 520, while the Hk of the hardmagnetic layer 102 is relatively low. As the temperature continues todrop, the Hk of the hard magnetic layer 102 increases until it reachesfreezing temperature, as indicated by vertical line 522, at which pointswitching no longer takes place. At the freezing temperature, themagnetization of the hard magnetic layer 102 is locked, and will nolonger change in the presence of the applied magnetic field. Thevertical exchange coupling between hard layer 102 and soft layer 104,enhanced by ECL 106 increases the effective moment and thermal stabilityof hard layer 102 as well as increases the driving force for magneticrotation of hard layer 102; all of which increase the onset temperature520 at which the hard layer 102 begins to switch. This earlier highertemperature onset increases the temperature window and correspondinglythe likelihood that grains will switch as desired before the freezingtemperature is reached.

Referring now to FIG. 6, a graph 600 illustrating the effect oftemperature on Ms in various layers of a HAMR media is shown accordingto one aspect of the present embodiments. The graph 600 illustrates someembodiments wherein the ECL 106 (FIG. 1) includes a high Ms at roomtemperature, as indicated by the line Ms2 ECL, that is higher than boththe soft magnetic layer 104 (FIG. 1) as indicated by the line Ms3 Softand the hard magnetic layer 102 (FIG. 1) as indicated by the line Ms1Hard. FIG. 6 shows a case where Ms3 Soft is higher than Ms1 Hard, butthese relative values can be reversed in various embodiments.

The graph 600 also illustrates that in some embodiments the hardmagnetic layer 102 includes a Tc that is lower than both the softmagnetic layer 104 and the ECL 106. FIG. 6 shows a case where Tc3 Softis lower than Tc2 ECL, but these relative values can be reversed invarious embodiments. In various embodiments, the ECL 106 includes ahigher Ms than the soft magnetic layer 104 at the Tc of the hardmagnetic layer 102, and most importantly, in the range of temperaturesbetween the onset and freezing temperatures of the hard layer.

In various embodiments, it may be desirable for the ECL 106 to include alow sigma Tc. In some embodiments, the sigma Tc of the ECL 106 may belower than sigma Tc of the soft magnetic layer 104 and sigma Tc of thehard magnetic layer 102. In further embodiments, the segregantconcentration of the ECL 106 is lower than that of the hard magneticlayer 102 so as to increase Ms and reduce sigma Tc. In otherembodiments, the Fe:Pt ratio of the ECL 106 is closer to 1:1 than thehard layer, in order to improve ordering and reduce sigma Tc. In otherembodiments the ordering temperature of ECL 106 is lowered so as toincrease ordering and lower sigma Tc. Several elements including Cu andAg are known to reduce ordering temperature in FePt.

In the presence of an applied magnetic field (not shown), switchingonset of the magnetization in the hard magnetic layer 102 begins atsomewhat below the Tc of the highest Tc grains 108 of the hard magneticlayer 102, as indicated by vertical line 620, while the Ms of the hardmagnetic layer 102 is relatively low. Within the hard magnetic layer102, various grains may switch at slightly different Tc values. Forexample, some grains may begin to switch at higher temperatures and somegrains may not switch until a lower temperature is reached. Such avariation is indicated within the graph 600 by Sigma Tc. As a result,switching may begin at a range of temperatures below Tc+sigma Tc of thehard magnetic layer 102, and successful switching may occur for eachgrain during the period of time that the disk cools, extending over arange of cooler temperatures until the freezing temperature is reached.The success rate and corresponding transition quality depends on theamount of time and the strength of the driving force that directs thegrains to switch during this cooling process.

Referring now to FIG. 7, a graph 700 illustrating delayed switching ofthe hard magnetic layer 102 (FIG. 1) is shown according to one aspect ofthe present embodiments. Similar to FIG. 6, the graph 700 illustratessome embodiments wherein ECL 106 (FIG. 1) includes a higher Ms. In somepreferred embodiments room temperature Ms ECL is even higher than thesimilar embodiments of FIG. 6 so as to increase the rate of rise of MsECL and resulting vertical exchange coupling between hard layer 102 andsoft layer 104, as temperature is decreased below Tc ECL.

On the other hand, the graph 700 illustrates Tc values that aredifferent from FIG. 6. In various embodiments the ECL 106 includes a Tcthat is lower than the soft magnetic layer 104 and similar to or lowerthan the hard magnetic layer 102. The hard magnetic layer 102 includes aTc that is lower than the soft magnetic layer 104 and similar to orhigher than the ECL 106. The soft magnetic layer 104 includes a Tc thatis higher than both the hard magnetic layer 102 and the ECL 106. Invarious embodiments, the ECL 106 includes a lower Ms than the softmagnetic layer 104 at Tc+sigma Tc of the hard magnetic layer 102. Invarious embodiments, the ECL 106 may include a Tc value that is 0-50degrees lower than the hard magnetic layer 102. In further embodiments,hard magnetic layer 102 and/or the soft magnetic layer 104 may includeat least two sublayers. The sublayers may include different magneticvalues (e.g. Tc, sigma Tc, Ms, etc.). The ECL 106 may have a similar orlower Tc than a top sublayer of the hard magnetic layer 102, wherein theECL 106 is in direct contact to the top sublayer and a bottom sublayerof the soft magnetic layer 104. In some embodiments of ECL 106, Tc isreduced below that of hard layer 102 by including a higher concentrationof Tc reducing elements such as Cu and Ru. In such manner and incombination with reduced segregant or moment diluting elements, orincreased Fe or Co concentration, an ECL having higher verticalcoupling, lower Tc, and narrower sigma Tc than the top sublayer of hardlayer 102 can be achieved. Large sigma Tc of the hard magnetic layer 102may be damaging to recording performance. In various embodiments, it maybe desirable for the ECL 106 to include a low sigma Tc, particularly inembodiments where the Tc of the ECL 106 is less than the Tc of the hardmagnetic layer 102. The ECL 106 may include Ag to lower the orderingtemperature and improve sigma Tc. In various embodiments, it may bedesirable for the ECL 106 to include a very high Ms, particularly inembodiments where the Tc of the ECL 106 is less than the Tc of the hardmagnetic layer 102. In such embodiments, the onset temperature of thehighest Tc grains of the hard layer is relatively reduced because theECL does not couple the hard layer to the soft layer above Tc ECL, whilelowest Tc grains of the hard layer have relatively higher onsettemperature because coupling between the hard and soft layers is turnedon near Tc ECL, which is closer to Tc hard − sigma than it is to Tchard + sigma.

In the presence of an applied magnetic field (not shown), switchingonset of the magnetization in the hard magnetic layer 102 no longerbegins at the Tc of the hard magnetic layer 102. Instead, switchingonset is delayed toward the Tc of the ECL 106, at a temperature belowthe Tc of the hard magnetic layer 102. Such a delayed switching onsetincreases uniformity of switching. Sigma T onset can be made lessdependent upon sigma Tc hard, and thus be reduced to a narrower range.

While the embodiments have been described and/or illustrated by means ofparticular examples, and while these embodiments and/or examples havebeen described in considerable detail, it is not the intention of theApplicants to restrict or in any way limit the scope of the embodimentsto such detail. Additional adaptations and/or modifications of theembodiments may readily appear, and, in its broader aspects, theembodiments may encompass these adaptations and/or modifications.Accordingly, departures may be made from the foregoing embodimentsand/or examples without departing from the scope of the conceptsdescribed herein. The implementations described above and otherimplementations are within the scope of the following claims.

What is claimed is:
 1. An apparatus comprising: a first magnetic layer;a second magnetic layer overlying the first magnetic layer, wherein thesecond magnetic layer is magnetically softer than the first magneticlayer; and an exchange control layer between the first magnetic layerand the second magnetic layer, wherein the exchange control layer ismagnetic and increases vertical coupling between the first magneticlayer and the second magnetic layer, and the exchange control layerincludes a lower sigma Tc than the first magnetic layer.
 2. Theapparatus of claim 1, wherein the first magnetic layer, the secondmagnetic layer, and the exchange control layer are granular.
 3. Theapparatus of claim 1, wherein the exchange control layer includes ahigher magnetic moment Ms than the first magnetic layer and the secondmagnetic layer.
 4. The apparatus of claim 1, wherein the exchangecontrol layer includes a higher Fe concentration than the first magneticlayer.
 5. The apparatus of claim 1, wherein the exchange control layerincludes a lower segregant concentration than the first magnetic layer.6. The apparatus of claim 1, wherein the first magnetic layer and theexchange control layer include Fe—Pt and the exchange control layerincludes Co.
 7. An apparatus comprising: a first magnetic layer; asecond magnetic layer overlying the first magnetic layer, wherein thesecond magnetic layer is magnetically softer than the first magneticlayer; and an exchange control layer between the first magnetic layerand the second magnetic layer, wherein the exchange control layer ismagnetic and increases vertical coupling between the first magneticlayer and the second magnetic layer, and the exchange control layerincludes a higher magnetic moment Ms than the first magnetic layer andthe second magnetic layer.
 8. The apparatus of claim 7, wherein thefirst magnetic layer, the second magnetic layer, and the exchangecontrol layer are granular.
 9. The apparatus of claim 7, wherein theexchange control layer includes a higher Fe concentration than the firstmagnetic layer.
 10. The apparatus of claim 7, wherein the exchangecontrol layer includes a lower segregant concentration than the firstmagnetic layer.
 11. The apparatus of claim 7, wherein the first magneticlayer and the exchange control layer include Fe—Pt and the exchangecontrol layer includes Co.
 12. An apparatus comprising: a first magneticlayer; a second magnetic layer overlying the first magnetic layer,wherein the second magnetic layer is magnetically softer than the firstmagnetic layer; and an exchange control layer between the first magneticlayer and the second magnetic layer, wherein the exchange control layeris magnetic and increases vertical coupling between the first magneticlayer and the second magnetic layer, and the exchange control layerincludes a lower segregant concentration than the first magnetic layer.13. The apparatus of claim 12, wherein the first magnetic layer, thesecond magnetic layer, and the exchange control layer are granular. 14.The apparatus of claim 12, wherein the exchange control layer includes ahigher magnetic moment Ms than the first magnetic layer and the secondmagnetic layer.
 15. The apparatus of claim 12, wherein the exchangecontrol layer includes a higher Fe concentration than the first magneticlayer.
 16. The apparatus of claim 12, wherein the first magnetic layerand the exchange control layer include Fe—Pt and the exchange controllayer includes Co.